Modern hot-melt type thermoplastic adhesives are useful for permanently bonding a wide variety of components. They have excellent thermal and physical properties which makes their use in a manufacturing assembly process highly desirable. Generally, hot-melt adhesives are made from a thermoplastic material which softens when heated. The softened adhesive is very tacky and adheres well to a variety of substrates. As the adhesive cools, it hardens and forms a rigid bond.
The adhesive is generally preheated by special heaters to increase its viscosity from a solid or near solid consistency to a paste or liquid consistency. The softened adhesive is applied between matting surfaces of pieces to be bonded and the adhesive allowed to cool. Once the adhesive cools, the pieces form a permanent assembly. It is very difficult to remove a part from the assembly. The assembly may be reheated to a temperature sufficient to soften the adhesive and thus remove the part, but this may require heating the entire assembly in an oven. The matting surfaces between the parts are often inaccessible to a heat source. It is difficult to apply heat only to the adhesive and not the adjacent parts. If one or more of the adjacent parts are heat sensitive, they may be damaged by the heat necessary to soften the adhesive. Additionally, hot-melt adhesives require special mixing, heating and applying apparatuses to assure the adhesive is at the proper temperature when applied. Hot-melt adhesives are generally not heated after they are applied to the parts. This subsequent heating is known to cure thermosett adhesives.
Thermosett adhesives require additional heat not to soften and become tacky, but to aid in achieving cure. They differ from hot-melt adhesives because they undergo a chemical reaction which cross-links molecules in the adhesive. Once cured, thermosett adhesives cannot be reheated to become soft.
Several common means of heating thermosett adhesives include conduction, convection and radiant heating. Conduction heating is achieved by placing a wire in the adhesive and passing a current through the wire. The current heats the wire which in turn heats the adhesive. Conduction heating has the disadvantage of requiring a conductive wire to be placed in the adhesive bond. The wire is difficult to be properly located within the adhesive and weakens the bond.
Radiant or convection heating is achieved by placing the part to be bonded with adhesive in a radiant or convection oven. The adhesive is heated indirectly by heat passing through the parts to be bonded. Convection or radiant heating requires that both the parts and the adhesive be heated in an oven. Some parts, such as thermoplastics, may weaken or deform when heated to the temperatures necessary to cure the adhesive. Further, radiant and convection heating generally require upwards of twenty minutes to cure the adhesive.
Another method of curing thermosett adhesives by concentrating heat within the adhesive is taught and disclosed in U.S. Pat. No. 4,749,833, issued Jun. 7, 1988 to Novorsky et al. Novorsky et al. teaches induction heating of an adhesive. Spherical particles of steel are placed within the adhesive and moved in rolling contact with one another to establish an accurate space between two members to be bonded in an adhesive joint. The adhesive joint is placed between or adjacent to an induction coil and a current is passed through the coil. The current passing through the coil induces spherical particles to heat thereby heating the surrounding adhesive. As in the method of conduction heating, induction heating also requires the addition of foreign metal particles within the adhesive, thereby decreasing the strength of the bond.
It is known to heat polar materials including thermosett adhesives in a high frequency electric field by a process called dielectric heating. U.S. Pat. No. 3,291,671, issued Dec. 13, 1966 to Hecht, teaches a fusing of plastic films by dielectric heating. A water containing paper board separated by one or more polyethylene films is placed between two electrodes. A radio frequency (RF) generator is attached to the electrodes and passes an electric field through the polyethylene film and water containing paper board. The polyethylene films fuse to one another and to the paper board. This illustrates dielectric heating but not the dielectric heating of an adhesive to achieve curing.
Dielectric heating to cure thermosett adhesives is shown and disclosed in EPO Patents 0,339,494 and 0,339,493, both filed Apr. 20, 1989 and U.S. Ser. No. 07/187,358 filed May 28, 1988. These patents teach the bonding of a fiber-reinforced plastic (FRP) exterior member to a U-shaped FRP reinforcement member. A bead of two-part epoxy resin adhesive is placed between the exterior and reinforcement members. The bonded assembly is then moved to a chamber containing a dielectric heater. The first electrode having roughly the same contour as the outer skin is placed against the outer skin member and a second electrode having a concentrator is placed over the reinforcement member. A high-frequency electrostatic field of between 300 and 8,000 volts is applied through the electrodes. The high frequencies range between 25 and 40 MHz. This apparatus teaches curing times of approximately 30 to 40 seconds. The apparatus uses a plurality of electronically isolated concentrators overlying the exterior member. After the exterior member is bonded to the reinforcement member, the concentrators are removed. The EPO patents relate specifically to thermosett adhesives and require separate electrodes for both parts being bonded. They do not teach the use of an integral electrode, or the use of a conductive frame as an electrode. Additionally, the EPO patents do not teach the heating of hot-melt adhesives or the disassembly of a bonded assembly.
When bonding elongated parts such as instrument panels, it is difficult to access mating surfaces between the elongated part and frame. Commonly, instrument panels are installed before the window glass and access to the frame can be achieved through the window opening. Most instrument panels use mechanical fasteners such as screws or bolts to secure the panel to the frame. The problem with mechanical fasteners is that loosening of the screws causes squeaking and rattling the instrument panel. Bonding the instrument panel directly to the frame alleviates the squeaking and rattling. Conventional bonding with thermosett adhesives makes removal of the instrument panel difficult. Instrument panel removal is believed of much greater concern with the advent of in panel air bags. By teaching the removable bonding of an instrument panel to a frame, the difficulties of mechanically fastening the instrument panel as well as the problems of squeaking and rattling are overcome.
Similar problems occur when mechanically fastening a lens to a lamp assembly. Mechanical fasteners provide a less attractive finished appearance and require additional sealant around the lens perimeter. Conventionally bonded lens cannot be easily removed from the lamp assembly without damaging the fragile lens. By providing a method and device for removal by bonding a lens to a conductive frame, lamp assemblies may be designed for bulb replacement from the exterior of the vehicle. Exterior access for bulb replacement simplifies packaging of the lamp assembly as well as sealing the lens.
It is a primary object of this invention to utilize RF dielectric heating for removably bonding a non-conductive member to a conducting frame. The invention permits the removable bonding of elongated parts such as an instrument panel where it is otherwise difficult to fasten to the frame. An electrode is permanently secured to the non-conductive member and can be used to secure portions which are not otherwise easily accessible. The need for separate electrodes is eliminated by using the conductive frame as one electrode and permanently attaching the other electrode to the bonded part. Nonuniform heating of elongated plates is reduced by pulsing the electric field.